Decided to use my grandfathered Verizon 3G unlimited account to test the Chromecast. Ran into a small hick-up trying to set it up - (I use an old Verizon(Pantech) UM-175VW USB modem plugged into a Cradelpoint MBR-1000 router.)
Using the Cradelpoint MBR1000 wireless router (And I suspect the 350) - When connecting a Google Chromecast to the router, it connects fine but shows on the TV an error that it is connected to the router but cannot access the internet. This is because the MBR-1000, by default, requires confirmation of Internet Access Authorization by entering the ADMIN password the first time a connected device attempts to access the internet.
However, since Chromecast is just a HDMI dongle there is no way to enter a password through chromecast (Nor even see the P/W entry screen normally sent by the MBR-1000.)
This "required authentication" feature of the MBR-1000 is a toggle and can be turned off without making the router an open network. Log into the MBR-1000 as administrator, Under Basic select Setup Wizzard, on tyhe second screen - un-check the box where it says require authentication for internet access. Finish the Wizard and allow the router to reboot. Chromecast will suddenly access the Internet and give you "Ready to Cast"
As to it's use under 3G - Seems to work fine. Only a couple short buffering occurrences in 1/2 Hr viewing when watching a HD 1020P Netflix movie and I am accessing the internet in parallel on the laptop.
So far - So good! Since Vzn meters my usage even under the unlimited I'll be able to see how much bandwith I actually consume with this thing.